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So-called health tourists cost the NHS somewhere between £70 million and £300 million each year, according to a new study.
But the Health Secretary Jeremy Hunt denied this was "a sketchy gap" in the numbers and defended the "very thorough research".
He said: "The worst thing would be if the research invented numbers that couldn't be backed up so they've quite properly given a range."
Labour's shadow health secretary Andy Burnham said: "We are in favour of improving the recovery of costs from people with no entitlement to NHS treatment.
"But it's hard not to conclude that this announcement is more about spin than substance. The Government's own report undermines their headline-grabbing figures, admitting they are based on old and incomplete data.
"Instead of grandstanding, the Government need to focus on delivering practical changes. Labour would not support changes that make doctors and nurses surrogate immigration officials."
Health Secretary Jeremy Hunt said that the NHS is a "national health service - not an international one".
Mr Hunt has pledged to identify a "more efficient system" of claiming back costs and has also appointed Sir Keith Pearson to advise on visitor and migrant cost recovery.
He has also said he will be "introducing a simpler registration process to help identify earlier those patients who should be charged".
Mr Hunt said the report shows there is a "serious problem", adding: "Having a universal health service free at the point of use rightly makes us the envy of the world, but we must make sure the system is fair to the hard-working British taxpayers who fund it."
The figures have been released ahead of the second reading of the Government's Immigration Bill, which aims to bring in measures to stop migrants abusing public services and make it easier to remove people who should not be here.
Key measures in the Bill will see temporary migrants, such as overseas students, pay to access the NHS.
Ministers hope the levy on students or foreign workers who come to the UK for more than half a year will generate £200 million a year.
The surcharge, combined with better recovery of costs and a deterrent on health tourism could save the health service "well over half a billion pounds", the Department of Health spokeswoman said.
The NHS could claw back more than £500 million a year if it was better at charging foreign nationals for using its services, a report suggests.
The health service could raise the cash - which is the equivalent of 0.45% of its annual budget - by deterring so-called "health tourists", recovering care charges from overseas visitors and charging temporary migrants for accessing NHS care, the report found.
Health tourists - such as women who travel to the UK in late pregnancy and give birth in an NHS hospital before returning home - cost the health service at least £70 million each year, according to the study on migrant access to NHS care in England.